The present invention relates to improvements in the efficiency of internal combustion engines within combined-cycle power plants.
Combined-cycle power plants are known and are becoming dominant in the larger engine power industry, particularly in fixed-plant applications. Commonly, these power plants include gas turbine generators exhausting to Rankine cycle steam generating plants and are found in large installations with gas turbine generators rating as high as 390 megawatts with a claimed thermal efficiency of 58% for the combined cycle. Conventional gas turbines consist of a bladed compressor and a bladed expander mounted on the same shaft. The compressor, as distinguished from positive displacement engines, must run at high RPM to pump air at low pressure. The resulting large mass flow of high temperature air requires large heat recovery equipment. Systems of this type generally operate continuously at full load, because both efficiency and torque drop considerably with a reduction of size, speed or load.
The configuration of a screw engine is comparable to that of a gas turbine to the extent that both include a compressor, a combustor, and an expander. Largely for this reason, screw engines are commonly mis-identified as screw turbines when, in fact, they are positive displacement mechanisms comparable to a piston engine. The close clearances of screw engines make them self-cleaning, free of the deposits that build up in the bladed compressor of a gas turbine. Air-fuel ratios can be maintained at optimum levels over the range of operation, so there is no excess air and mass flow. The result is higher exhaust temperatures, permitting the use of smaller heat recovery units. Further, the expander may be equipped with an expansion ratio modulation system, as is commonly known in screw engine applications. Under low load conditions, there could be an over-expansion of the gasses, resulting in a power drag on the unit. A capacity control modulation system results if slots are cut in the first compressor stages. Opening a slide valve vents these stages to the inlet end of the unit to delay compression. Similar slots in the screw expander at the exhaust end give similar early exhaust at low loads.
Combined-cycle power systems are also found in vehicles such as large trucks, locomotives and busses. Regenerative vehicle braking systems have been developed using flywheels, where the braking energy speeds up a flywheel to store energy which is later used to propel the vehicle. The Swedish Cumulo system uses braking energy to pump oil into a chamber at up to 6,000 psi. The pressure energy is then used to accelerate the vehicle. This system requires heavy duty piping and components which reduce the vehicle""s payload carrying capacity. Electric drive power systems, generally known as hybrid systems, are also known. An engine drives a generator, which in turn powers the electric drive motor. On braking, computer control changes the drive motor to generator mode and electricity is fed back into a battery grid. This system has a limited amount of energy storage and, when the storage limit is reached, further braking energy is wasted.
Current submarine propulsion systems have unique problems due to the desirability of remaining submerged for long periods of time while retaining the capability of moving at high speeds. At present, the world naval submarine fleet numbers almost seven hundred boatsxe2x80x94some of which are nuclear. Nuclear submarines have the ability to submerge and stay under water for weeks or even months. However, they are large, heavy, and very costly to build and operate. They are also designed to meet the Cold War need for difficult-to-detect, deep ocean, strategic nuclear weapons platforms. Because of their size, they are not suitable for the littoral warfare foreseen for the present and near future.
The great majority of the world""s submarines have non-nuclear diesel-electric propulsion systems. For the cost of a nuclear submarine, four or more diesel-electric submarines could be built which would be equal to or better in agility, maneuverability, and quietness than nuclear submarines. Since the advent of the submarine, however, designers have been faced with the problem that the conventional non-nuclear submarine required two power systems for propulsionxe2x80x94internal combustion engines for surface use and battery charging, and battery systems when submerged. Diesel-electric submarines must surface (at least to periscope depth) often, depending upon their use of battery power. Surfacing to charge batteries takes time, during which the submarine is most vulnerable to detection.
In order to extend submerged time for diesel-electric submarines, various air-independent propulsion (AIP) systems have been developed. These systems are generally not over three hundred horsepower and commonly only extend the use of the batteries. They could be used directly to provide propulsion power, but only at relatively slow speed. Higher speeds would drain the propulsion batteries, and flank speed would likely drain the batteries within a few hours. Present AIP systems are offered for retrofitting in older boats. To accommodate the system, a plug, equal to the diameter of the hull, must be installed in the submarine. This makes the submarine heavier, longer, and less maneuverable. AIP systems in the prior art include Stirling engines, MESMA systems, fuel cells, and closed cycle diesels.
The Stirling engine is used in the Swedish Kockums design. The design uses two or more Stirling engines, which require a special fuel oil, as well as liquid oxygen (LOX), naphtha for the AIP, and diesel fuel. The French MESMA system uses a simple Rankine cycle, with a high consumption of fuel oil and LOX. The steam pressure generated is approximately 260 psi and the closed combustion pressure is approximately 870 psi, which makes it possible to blow the exhaust overboard at great depth. Both the Swedish and the French systems operate on a closed cycle with continuous combustion. Resulting carbon dioxide (CO2) is not detrimental to the combustion process and is used as the working fluid. Other combustion products include water and other non-combustible gasses.
The fuel cell has been touted as the power system of the future, for both vehicle and marine power. It has a number of problems, including high weight/horsepower ratio and high fabrication costs due to utilization of costly materials. Submarines equipped with fuel cells must carry fuel oil for the diesels as well as LOX and hydrogen for the fuel cells. The hazards associated with hydrogen make this a questionable material to be carried on a military vessel. Finally, thermal efficiency at low speeds falls off as speed and power demands increase. Balancing these problems is the absence of exhaust pollution, for the sole fuel cell by-product is pure water.
The addition of any of the above AIP systems makes it possible for a submarine to remain submerged for weeks at a time provided they are operated at 4-5 knots and the main propulsion batteries are not drawn upon. At any higher speed, main batteries must be used, and the submarine may have to surface several times in one day to charge the batteries. These AIP systems add to the length and weight of the submarine in new construction, and in retrofitting an older boat the previously mentioned plug is required. For example, the French MESMA system adds 270 horsepower to the submarine, yet adds 250 tons to its weight and 33 feet to its length. The result includes increased water resistance under speed as well as a reduction in maneuverability.
The closed cycle diesel system is the only proven AIP system that propels the submarine both on the surface and when submerged. The system requires diesel fuel, oil, oxygen and argon for submerged closed-cycle operation.
All of the systems described above except the fuel cell generate carbon dioxide as a by-product. A closed cycle diesel must remove the carbon dioxide from the working fluid because carbon dioxide delays fuel combustion. Exhaust generally passes through an absorber where sea water removes the carbon dioxide (CO2) and the remaining gas returns to the closed cycle. The problem may be further remedied by the injection of argon gas. This is not necessary in the MESMA system and the Stirling engine because their fuel combustion is constant and there is time for adequate combustion. MESMA and Stirling AIPs commonly use carbon dioxide as the working fluid.
The prior art reveals many deficiencies which would benefit from a significant improvement in the design and efficiency of a combined internal combustion and electrical power system which can be utilized in open, closed, or semi-closed cycle modes.
Accordingly, it is a primary object of the invention to significantly improve the energy efficiency of combined cycle power plants by incorporating an internal combustion engine and a Brayton Bottoming System. The system recovers heat from the exhaust of internal combustion engines by means of an open or closed bottoming cycle utilizing the compression or expansion of the working fluid. The internal combustion engines are preferably of the screw design. Although a screw engine is most suitable, any internal combustion engine with a hot exhaust could be used. Alternatives include a Wankel-type rotary engine, a Sterling engine, or a gas turbine. The secondary system is preferably a Brayton Bottoming System (BBS) in which compressed gaseous fluid is pumped through a heat exchanger and then expanded utilizing screw expanders and a positive displacement screw compressor with a modulation control valve to permit variable speed operation. With the inclusion of energy storage in a thermal battery, the system would have an attractive power to weight ratio, with a high power density. Ambient air would likely be utilized in an open cycle such as would be used with road vehicles. In a closed cycle, such as in submarines or in a mine vehicle, the working fluid might be an inert gaseous fluid such as nitrogen or carbon dioxide.
In a preferred embodiment, the engine is either a turbine or piston-driven type. Turbine rotors are preferably made of high temperature material, and the engine may be considered an adiabatic or near-adiabatic engine. Mass flow is reduced by building a turbine and a screw compressor with both the compressor and the turbine on the same shaft, or with a reduction gear. Small turbines have a compression ratio of about 3.5:1, while a screw compressor would make possible a ratio of 30:1. Either of these configurations would have continuous combustion, which would have environmental benefits and would be quieter than a diesel engine. Many critical engine components such as bearings are made of self-lubricating, non-magnetic ceramic. Cerbic or Noralide bearings made by Norton are commercially available and withstand temperatures of 1832xc2x0 F. and 2370xc2x0 F., respectively. Use of such bearings allows the lubrication system to be minimized or dispensed with entirely.
For vehicle applications, regenerative braking is used to pump compressed air to a high pressure storage tank. In run mode, this compressed air is routed through the heat exchanger and then expanded to assist in propelling the vehicle. In an electric propelled vehicle in regenerative braking mode, the drive motor can change to generator mode and use the kinetic energy of the braking vehicle to heat the thermal battery in the heat exchanger to further heat the air or other gaseous fluid to be expanded. The two methods of regenerative braking are computer controlled and may be used either separately or in tandem. With the first method, braking causes the computer to convert a drive motor to generator mode. The resulting electrical energy is used to heat a high capacity thermal battery. With the second method, the main drive motor, when operated in generator mode, pumps pressurized gaseous fluid into a storage tank. Upon returning to drive mode, this compressed gaseous fluid is led through a thermal heater and then expanded to produce drive power for the vehicle.
The invention in closed cycle mode is particularly suitable for marine applications, which would not utilize regenerative braking. Naval applications, including mine sweepers and submarines, would preferably employ an adiabatic engine of non-magnetic ceramic construction. A screw engine is favored because of its inherent high power density, high temperature exhaust, and quieter continuous combustion.
Accordingly, it is an object of the present invention to provide a power system that will propel a submarine at full speed, whether surfaced or submerged, and that will permit the vessel to remain submerged for the greater or whole part of a mission, the power system running on fuel oil and oxygen supplied from a store of liquid oxygen.
It is also an object to utilize regenerative braking to maximum efficiency in wheeled vehicles having combined-cycle propulsion.
It is a further object to utilize a thermal battery to improve the efficiency of a combined-cycle power plant.
It is a still further object to improve the efficiency of combined cycle power plants by combining a screw turbine with a Brayton Bottoming System.
It is also an object to operate a turbine at variable speeds without significant power loss.
An engine having these and other advantages includes an internal combustion engine driving a first motor/generator; a heat exchanger; means connecting the engine and heat exchanger for providing heat generated by the engine to the heat exchanger; a first expander driving a second motor/generator; means connecting the heat exchanger and the first expander for providing heat from the heat exchanger to the first expander; a third motor/generator; and means for providing power between at least one of the first and second motor/generators to the third motor/generator.